The long-term objective of this research is to understand how organisms evolve at a molecular level. The short-term goal is to understand gonad development in the nematode Pristionchus pacificus. The nematode gonad is a complex organ composed of both somatic and germline tissues. To form a functional organ, the right number of cells must be made, the right cell-types specified, and ceils must rearrange to form functional tissues of the right size in the correct location. Many of these processes are controlled by similar pathways and molecules in all organisms, whether worm or human. When these pathways are aberrantly controlled, the results are often detrimental. However, changes in developmental processes must occur to allow for the generation of species. It is not known how plastic fundamental processes are. Much effort has established P. pacificus as a second genetic model nematode for comparison with C. elegans. Investigation of gonad development in P. pacificus offers the opportunity to study change in many developmental processes through comparisons. The gonad of P. pacificus has gross morphological differences in comparison to C. elegans. Over the next three years, the P. pacificus gonad will be characterized and genes involved in gonad development will be isolated. Classical lineaging techniques and laser ablations will be used to fate map the gonad. Cell contacts will be further described using electron micrographs. Concurrently, unbiased genetic screens will uncover involved loci. Mutants will be classified by phenotype and according to the characterization of the gonad and inferred cell interactions. Comparison with C. elegans will begin to dissect how the processes that form the nematode gonad and germ line have changed. Ultimately, insights into the pattern of molecular changes that can occur and may have resulted in the varied metazoans we know may be gained.